\begin{tikzpicture}[>=Stealth]
    \node at (0,-0.3) {$O$};
    \draw (0,0) -- (3,0) node [anchor=west] {$A$};
    \draw [rotate=135] (0,0) -- (3,0) node [anchor=south] {$B$};
    \draw [->] (0.5,0) arc (0:135:0.5);
    \node at (0.5, 0.5) {$\alpha$};
\end{tikzpicture}
